1. Field of the Invention
The present invention relates to a ground fault sensing device.
2. Description of Related Art
Since, in an electric vehicle or a hybrid electric vehicle, a high voltage battery is provided for operating devices such as an electric motor and an inverter, accordingly it is necessary to provide a ground fault sensing means for protecting the personnel riding in the vehicle from this high voltage. Due to this, technical standards have been determined for protecting the personnel riding in the vehicle from high voltage. For example, it has been made a requirement to install a device that monitors the insulation resistance between the active electrode and the electrical chassis, and that issues a warning to the driver when the value of this insulation resistance drops to 100 Ω per one volt of operating voltage.
With a prior art ground fault sensing device, principally, what is detected is the occurrence of a ground fault on the direct current side of the high voltage battery (for example, refer to Patent Document #1). Thus, with the ground fault sensing method described in Patent Document #1, a ground fault sensing signal is supplied to a direct current power supply circuit via a detection resistor and a coupling capacitor. And the amplitude of the voltage at a ground fault sensing point, which is the point of connection between the detection resistor and the coupling capacitor, is sampled at a sampling cycle that is ½ of the cycle of this periodic waveform (i.e. of the ground fault sensing signal), and determination of whether or not a ground fault is occurring is made by obtaining the difference between the voltage amplitude values detected during odd numbered sampling cycles and even numbered sampling cycles.
Patent Document #1: Japanese Patent Publication No. 3678151.
However since, with the method disclosed in Patent Document #1, it is arranged to detect the occurrence of a ground fault on the battery side (i.e. the direct current side), accordingly it is difficult to detect the occurrence of a ground fault on the electric motor side (i.e. the alternating current side). Furthermore, sometimes it may happen that the ground fault determination described above becomes difficult if a ground fault has occurred upon the alternating current side, because an amplitude wave corresponding to the rotation speed of the electric motor is superimposed via the Y-capacitor upon the periodic waveform.